Photochemical activation of ruthenium(II)-pyridylamine complexes having a pyridine- N -oxide pendant toward oxygenation of organic substrates

Ruthenium(II)-acetonitrile complexes having η ³-tris(2- pyridylmethyl)amine (TPA) with an uncoordinated pyridine ring and diimine such as 2,2′-bipyridine (bpy) and 2,2′-bipyrimidine (bpm), [Ru ^II(η ³-TPA)(diimine)(CH ₃CN)] ²⁺, reacted with m-chloroperbenzoic acid to afford corresponding Ru(II)-acetonitrile complexes having an uncoordinated pyridine-N-oxide arm, [Ru ^II(η ³-TPA-O)(diimine)(CH ₃CN)] ²⁺, with retention of the coordination environment. Photoirradiation of the acetonitrile complexes having diimine and the η ³-TPA with the uncoordinated pyridine-N-oxide arm afforded a mixture of [Ru ^II(TPA)(diimine)] ²⁺, intermediate-spin (S = 1) Ru(IV)-oxo complex with uncoordinated pyridine arm, and intermediate-spin Ru(IV)-oxo complex with uncoordinated pyridine-N-oxide arm. A Ru(II) complex bearing an oxygen-bound pyridine-N-oxide as a ligand and bpm as a diimine ligand was also obtained, and its crystal structure was determined by X-ray crystallography. Femtosecond laser flash photolysis of the isolated O-coordinated Ru(II)-pyridine-N-oxide complex has been investigated to reveal the photodynamics. The Ru(IV)-oxo complex with an uncoordinated pyridine moiety was alternatively prepared by reaction of the corresponding acetonitrile complex with 2,6-dichloropyridine-N-oxide (Cl ₂py-O) to identify the Ru(IV)-oxo species. The formation of Ru(IV)-oxo complexes was concluded to proceed via intermolecular oxygen atom transfer from the uncoordinated pyridine-N-oxide to a Ru(II) center on the basis of the results of the reaction with Cl ₂py-O and the concentration dependence of the consumption of the starting Ru(II) complexes having the uncoordinated pyridine-N-oxide moiety. Oxygenation reactions of organic substrates by [Ru ^II(η ³-TPA-O)(diimine)(CH ₃CN)] ²⁺ were examined under irradiation (at 420 ± 5 nm) and showed selective allylic oxygenation of cyclohexene to give cyclohexen-1-ol and cyclohexen-1-one and cumene oxygenation to afford cumyl alcohol and acetophenone.